Slow Ascent

Janos · November 27th, 2005, 09:09 PM

Why ascend slowly from 6m to the surface?

Decompression theory is not an exact science, and you’re never going to be able to predict whether or not someone is going to have a bend or not, but I believe that by studying some of the theories currently circulating, I can understand what’s happening in my body a little better.

One of the questions that was troubling me (and when I say troubling – don’t worry – it wasn’t keeping me awake at night) is why are slow ascents on oxygen considered to be a good thing? Empirically some people say that they feel better after a slow ascent, but you can’t really draw conclusions from this, as this could be explained by many other things, and really we should be comparing a slow ascent with an extra 6 minutes at 6 metres.

I had no problems understanding why a slow ascent is a good idea if you’re breathing backgas or decoing out on Nitrox 50. Ascending will decrease the partial pressure of the inert gas in your lungs, hence there will be a larger “concentration gradient” between the inert gas in your tissues and the inert gas in your lungs, and so you will off-gas quicker.

However this doesn’t apply if you’re decoing out on pure oxygen. The partial pressure of inert gases in your lungs will be zero, and so I reasoned that the rate at which you off-gas would be independent of depth. So if it’s independent of depth, why bother with a slow ascent or a stop at 3 metres? Why not just stay at 6m, where you’re less likely to be affected by wave action above you.

I posted the question to the forum, here:

http://www.direxplorers.com/dir-tech...op-oxygen.html

Several people replied, but it seems that the current theory (I’m trying to shy away from saying “correct” or “right” when talking about deco!) is that when you ascend, the reduction in ambient pressure causes some of the inert gas to come out of solution as tiny bubbles. Small bubbles aren’t a problem, as they are quickly filtered out by the lungs. Also bubbles are inherently unstable, large bubbles tend to grow much larger, absorbing gas from the blood supply, whereas small bubbles tend to shrink quite rapidly and disappear. So keeping them small is definitely A Good Thing.

The best way to do this is to ascend slowly. As far as I can see, this has two effects: Not only keeping the bubbles small, but also reducing the speed at which they are produced, thus not overwhelming the lungs. Both of which are to be encouraged from a decompression point of view.

Janos

Attached Images:

Decograph.gif (3.4 KB, 0 views)

November 27th, 2005, 09:09 PM	#1 (permalink)
Janos(Offline) "Two Sheds" Join Date: Nov 2005 Location: Surrey Posts: 400	Slow Ascent Why ascend slowly from 6m to the surface? Decompression theory is not an exact science, and you’re never going to be able to predict whether or not someone is going to have a bend or not, but I believe that by studying some of the theories currently circulating, I can understand what’s happening in my body a little better. One of the questions that was troubling me (and when I say troubling – don’t worry – it wasn’t keeping me awake at night) is why are slow ascents on oxygen considered to be a good thing? Empirically some people say that they feel better after a slow ascent, but you can’t really draw conclusions from this, as this could be explained by many other things, and really we should be comparing a slow ascent with an extra 6 minutes at 6 metres. I had no problems understanding why a slow ascent is a good idea if you’re breathing backgas or decoing out on Nitrox 50. Ascending will decrease the partial pressure of the inert gas in your lungs, hence there will be a larger “concentration gradient” between the inert gas in your tissues and the inert gas in your lungs, and so you will off-gas quicker. However this doesn’t apply if you’re decoing out on pure oxygen. The partial pressure of inert gases in your lungs will be zero, and so I reasoned that the rate at which you off-gas would be independent of depth. So if it’s independent of depth, why bother with a slow ascent or a stop at 3 metres? Why not just stay at 6m, where you’re less likely to be affected by wave action above you. I posted the question to the forum, here: http://www.direxplorers.com/dir-tech...op-oxygen.html Several people replied, but it seems that the current theory (I’m trying to shy away from saying “correct” or “right” when talking about deco!) is that when you ascend, the reduction in ambient pressure causes some of the inert gas to come out of solution as tiny bubbles. Small bubbles aren’t a problem, as they are quickly filtered out by the lungs. Also bubbles are inherently unstable, large bubbles tend to grow much larger, absorbing gas from the blood supply, whereas small bubbles tend to shrink quite rapidly and disappear. So keeping them small is definitely A Good Thing. The best way to do this is to ascend slowly. As far as I can see, this has two effects: Not only keeping the bubbles small, but also reducing the speed at which they are produced, thus not overwhelming the lungs. Both of which are to be encouraged from a decompression point of view. Janos Attached Images: Decograph.gif (3.4 KB, 0 views) __________________ You can lead a horse to water but you can't climb a ladder with a large bell in both hands - Vic Reeves www.hellfins.com/shed